1. Field of the Invention
The present invention relates to a tuning circuit, more particularly the present invention relates to circuit constitution techniques for increasing Q by adding a negative resistance circuit to the tuning circuit in a receiver for using medium frequency band, short wave band, ultra-short wave band or so forth.
2. Description of the Related Art
Improvement of cross modulation characteristic of a receiver can best be done by improving frequency selectivity in a high-frequency wave section. However, efforts are generally made to increase Q of the tuning circuit as large as possible on the occasion of designing. For that reason, in most cases, it is difficult to further improve. Accordingly, there is no measure for improving the Q of the tuning circuit other than increase in the number of stage of the tuning circuit while adding high-frequency amplifier. However, it becomes necessary to interlock a plurality of tuning circuits with one another when increasing high-frequency amplifiers. Practically, it is very difficult to interlock a plurality of the tuning circuits with tuning frequency of the tuning circuit accurately made consistent over wide frequency band.
Accordingly, it may be possible to consider that attempt is made to increase Q of the tuning circuit by use of negative resistance circuit. The negative resistance circuit formerly was used in the Regenerative Detection receiver. In this Regenerative Detection system used here, it is possible to increase Q and gain in such a way as to approximately cancel resistance component that the tuning circuit originally possesses by use of the negative resistance generated caused by the positive feedback.
However, there occurs a problem when adopting the above-described Regenerative Detection system for increasing Q of the tuning circuit. In the Regenerative Detection system, oscillating state takes place when effective resistance becomes negative while passing resistance value zero. Therefore, operation should be made to set immediately before oscillating state. Further, as to this setting, different value of Q is adopted depending on the frequency. Thereby, complicated manual resetting should be performed in every channel selection.
The present invention has been made in consideration of the above-mentioned problem, and an object of the present invention is to provide a tuning circuit capable of always maintaining Q in high grade level even when changing tuning frequency of the tuning circuit.
In order to achieve the above mentioned object, according to a first aspect of the present invention, there is provided a tuning circuit in which an inductor, a variable capacitor, and a negative resistance circuit are connected in series, which tuning circuit comprises control means for controlling the negative resistance circuit so as to compensate a series resistance component of the inductor depending on change of tuning frequency by manipulations of the variable capacitor.
According to a second aspect of the present invention, in the first aspect, there is provided a tuning circuit, wherein the series resistance component consists of a fixed resistance component having nothing to do with a frequency and a variable resistance component whose resistance value is changed depending on a frequency, and a load resistance of the negative resistance circuit is constituted by a first and a second resistance components corresponding to the fixed resistance component and variable resistance component respectively, in which the control means controls the second resistance component so that the second resistance component changes depending on changes of the variable resistance component.
According to a third aspect of the present invention, in the first or the second aspect, there is provided a tuning circuit, wherein the negative resistance circuit is symmetry type multi-vibrator circuit, and the control means is a gain-polarity control section, in which a frequency control voltage which is applied to the variable capacitor is input to the gain-polarity control section.
According to a fourth aspect of the present invention, in the third aspect, there is provided a tuning circuit, wherein the gain-polarity control section comprises a first operational amplifier, an inverting amplifier, a second operational amplifier, a switch connected to both the first operational amplifier and the inverting amplifier, and a potentiometer connected to both the switch and the second operational amplifier.